In which scenario would you request air supply units?

Requesting air supply units is essential during the preparation for engine start because, at this phase, the aircraft systems need to be powered and operational to enable engine ignition and performance checks. Engine start procedures often demand that the air supply units provide the necessary bleed air to assist in the starting process, ensuring the engines can produce thrust effectively. Additionally, having adequate air supply allows other systems in the aircraft to function correctly and maintain cockpit and cabin environment standards.

In contrast, when engines are shut down, there is typically no need for external air supply since the engines do not require assistance for operation at that moment. During the boarding process, the aircraft might already be relying on its internal systems for climate control and air, rendering additional air supply units unnecessary. Similarly, when airstairs are in use, the focus is primarily on passenger access and egress rather than engine starting, and the need for supplemental air supply would not arise. Thus, the scenario that necessitates the request for air supply units is clearly during engine start preparations.